Bad miss on mag check then smooths out.

Roger Lee wrote:

How big an rpm drop? Plugs are usually bad or good and not intermittent. If it's a bad or severe miss it isn't a single bad  plug. Usually a bad plug, plug cap or wire is around 300 ish rpm drop

The miss shakes the engine real bad at 4000, one cylinder is not firing with one mag off.

Thinking out loud, I can reduce the problem to (4) plugs/wires/cap by which ignition system it is. Then reduce that to one side of the engine with EGT temp change. That will get me down to two possibilities. 

Is it shaking all the time below and above 4K rpm? How big a mag drop? Depending on all the factors it could also be a trigger coil? Maybe a trigger coil gap issue?

Roger Lee wrote:

Is it shaking all the time below and above 4K rpm? How big a mag drop? Depending on all the factors it could also be a trigger coil? Maybe a trigger coil gap issue?

I said 4000 rpm because that is what speed I use for mag check. It shakes at all speed, Definitely one cylinder not firing. I have to assume there is one plug not firing in a particular cylinder and when I turn off the other mag then that cylinder is not producing any power. 

SOLVED;

Many times I read about problems on this forum and nobody comes back to 'finish the story". Here is the rest of the story.

Today I ran the engine from cold 80f then when the oil hit120F I took the RPM to 4000 and performed a mag check. Again just like before I got a severe miss as though one cylinder was not firing. After trying a few different speeds the miss went away. Turning off the right mag switch was what caused the miss, that means I should be running on the following spark plugs #1T, #2T, #3B, #4B but one of those plugs isn't firing.

One of those plugs or wire was going to be my problem. Anything else is less likely because of commonality with another cylinder, or just not something that would only occur at a particular temperature then go away.

I started by cooling the engine down and trying again to confirm I still had the miss, it did miss and I immediately shut it off. I removed #1T plug wire and restarted. If the #1T was the one causing the miss then there would be no difference in the test. If #1T wasn't the one then the miss would have been a lot worse. On this run with #1T disconnected the miss was still there with no different intensity.

Now to isolate plug or wire. I swapped the #1T and #3T plugs, cooled the engine down and retested. This time the miss was there but it moved to the left mag. This means the miss followed the plug and not the wire (coil).

Conclusion; #1T plug is the cause of the miss.

Obviously I could have put in a new set of plugs and been all good but for me knowing the cause is important information.

The "bad" plug looks just fine, nothing different from past plugs with the same100 hours on them. The resistance on a random group of past used plugs are 3.8 - 4.1k ohms, this one is 4.8K.

Did you get the engine to smooth out?